The present invention relates to a liquid crystal cell using the electricity controlled birefringence effect and to processes for producing the cell and a negative optical anisotropy uniaxial medium usable therein. It more particularly applies to the production of data display devices, such as matrix screens and in particular to the construction of complex screens for colour display purposes.
Liquid crystal cells using the electrically controlled birefringence effect are already known. This effect has already made it possible to produce liquid crystal matrix screens and has already been described in publications such as the article by J. Robert entitled "T.V. image with L.C.D.", published in IEEE Transactions on Electron Devices, vol. ED 26, No. 8, August 1979 and the article by J. F. Clerc entitled "Electrooptical limits of the E.C.B. effect in nematic liquid crystals", published in the journal Displays, October 1981.
A liquid crystal cell using the electrically controlled birefringence effect e.g. comprises, according to the prior art, a nematic liquid crystal layer between two glass plates provided with transparent electrodes. Two polarizing means, e.g. two crossed rectilinear polarizers are respectively arranged on either side of the thus obtained assembly. When no voltage is applied between the electrodes, the molecules of the liquid crystal layer are substantially parallel to a direction called the "homeotropy direction" and perpendicular to the glass plates and incident light cannot pass through the cell. When an appropriate voltage is applied between the electrodes, the molecules of the liquid crystal layer are oriented substantially in a direction forming with the homeotropy direction an angle which is a function of the applied voltage or exciting voltage. An incident light can then at least partly pass through the cell. It is therefore possible to electrically control the light intensity transmitted through said cell, said intensity being a function of said voltage.
Liquid crystal cells using the electrically controlled birefringence effect suffer from the disadvantage that when observed obliquely, the contrast of these cells deteriorates and this increases as the observation angle increases and for certain observation angles this contrast can even be reversed.
French patent application No. 8407767 of May 18, 1984 proposes a liquid crystal cell using the electrically controlled birefringence effect and aiming at obviating this disadvantage.
However, the cell proposed suffers from disadvantages. It requires a given thickness of the liquid crystal layer, only permits an effective compensation of the birefringence of said layer in two incidence planes of the light and in the vicinity thereof and suffers from chromaticity defects, the extinction being less satisfactory for certain light wavelengths than for others.